The purpose of this application is to explore the cross talk that occurs between G protein-coupled receptors (GPCR's) and receptor tyrosine kinases (RTK's), especially the epidermal growth factor receptor (EGFR). We, like others, have made the observation that activation of GPCR's leads to transphosphorylation and activation of EGFR's. We have also made the novel finding that prior activation of GPCR's leads to a profound desensitization of the EGFR's. The purpose of this application is to gain mechanistic insights into the desensitization process. In Specific Aim 1, we will determine which signaling pathway(s) is/are required for desensitization and transphosphorylation of the EGF receptor by GPCR's. We will mainly use primary cultures of cell types that have endogenous GPCR's and EGFR's. Those studies will be augmented by judicious use of transfected cells. Major questions to be answered in this aim are: What signaling molecules link GPCR activation to EGFR phosphorylation and transactivation? Are they the same as those that induce EGFR desensitization? We will focus primarily on the roles of protein kinase C, the non-receptor tyrosine kinase Src, and sodium proton exchanger type I (NHE-I). Is endocytosis of EGFR required for GPCR-mediated desensitization of EGFR? Is heparin-bound EGF (HB-EGF) required for either desensitization or transactivation of the EGFR? In order to answer this question, we will use a number of complementary methods to neutralize the native HB-EGF in primary cells in culture. In Specific Aim 2, we will determine the intracellular fate of the EGFR's after stimulation of GPCR's. To what compartments are the EGFR targeted after transactivation by GPCR' s? What other signaling components are internalized with the EGFR (GPCR, NRTK, NHE-1, HB-EGF), and are they processed through the same pathways? What enzymatic pathways are required for down-regulation of the EGFR? We will study the roles of lysosomes, proteasome, and zinc-regulated metalloproteinases in this process. These studies address important gaps in our knowledge of GPCR signals that regulate the functions of RTK's.